WO2019053487A1

WO2019053487A1 - Stabilized laser or optical amplifier and stabilization method

Info

Publication number: WO2019053487A1
Application number: PCT/IB2017/055496
Authority: WO
Inventors: Kutan GÜREL; Thomas SÜDMEYER; Stéphane SCHILT
Original assignee: Université De Neuchâtel
Priority date: 2017-09-12
Filing date: 2017-09-12
Publication date: 2019-03-21

Abstract

A laser or amplifier optical device comprising a gain element (101, 32), a pump unit operatively arranged to pump the gain element into a condition of population inversion, wherein it can amplify a main optical signal with an amplification gain that is at least partially saturated, the optical device including an auxiliary input (22) optically connected to an auxiliary source (23, 113) generating an auxiliary optical signal that is propagated at least in a part of the gain element together with the main optical signal, and with a wavelength that is amplified in the gain element (101, 32), the optical device further comprising a control unit (118) operatively arranged to control the auxiliary source (23, 113)

Description

Stabilized Laser or optical amplifier and Stabilization method

Field of the invention

[0001] Embodiments of the present invention relate to the stabilization of lasers and optical amplifiers including, but not exclusively, fiber lasers and amplifiers. Among the applications of the invention are: the control of optical frequency combs; regulation of the carrier-envelope offset (CEO) frequency in lasers; active modelocking of lasers, stabilization of power and noise reduction in optical amplifiers.

Relevant art

[0002] Lasers and amplifiers are used in numerous applications. In several use cases, it is important to control their output properties such as pulse energy, wavelength, noise, or other. To this purpose, different control mechanisms have been developed and proposed.

[0003] It is known to change the optical power by using an acousto- optical modulator (AOM) or an electro-optical modulator (EOM). Such methods can be applied to continuous-wave (CW) and pulsed lasers, as well as to amplifier systems.

[0004] Pulsed lasers can provide optical frequency combs, which have been a breakthrough in diverse research field in physics since their first demonstration in the late 1990's. By providing a direct and phase-coherent link between optical and microwave frequencies, they enable the

measurement of optical frequencies with extreme precision, are an essential component in many novel atomic clocks with unprecedented frequency stability, and constitute a key element of ultra-low noise microwave oscillators, to name only a few established applications. New promising uses of optical combs are emerging in optical

telecommunications, broadband spectroscopy, and other fields. [0005] An optical frequency comb is generated by an ultrafast pulsed laser that emits a regular series of ultra-short light pulses, with a pulse duration typically shorter than 500 fs, in which all modes are phase- coherent (modelocked). This results in a comb-like optical spectrum made of discrete equally spaced lines. Each line has a frequency determined by just two radiofrequencies: the spacing between the lines, which

corresponds to the repetition rate f_rep of the laser pulse train, and the CEO frequency f_CEo, which is the frequency offset of the first comb line from the origin when the comb spectrum is extrapolated towards zero frequency. Physically, the CEO frequency results from the difference between phase and group velocities within the laser cavity, leading to a pulse-to-pulse phase slip between the pulse envelope and the underlying electric field. Stabilizing the two degrees of freedom of the frequency comb, i.e. the repetition rate and the CEO frequency, leads to a fully stabilized optical frequency comb in which each line has a known optical frequency v_N that is traceable to the SI second: v_N = N · f_rep + fcEo, where N is a positive integer that identifies the individual line in the comb.

[0006] The first demonstration of the stabilization of the CEO frequency in a modelocked laser that led to the first optical frequency comb was honored with the Nobel Prize in Physics in 2005. The CEO frequency can be stabilized via modulation of the pump power of the modelocked laser. This can be done by inserting an acousto-optical modulator in the pump beam, as it is conventionally done in Tksapphire lasers. The most commonly used combs today are based on ultrafast fiber lasers, e.g. using active fibers doped with Er, Yb, Tm, Ho, Nd, or Pd as gain material. These lasers are pumped by a diode laser, therefore the pump power can be

straightforwardly modulated by controlling the current of the pump laser. The achievable modulation bandwidth, however, is limited by the dynamics of the laser cavity, which strongly depends on the upper-state lifetime of the gain material, which is typically in the millisecond range for most common gain media. The present invention proposes, among others, a new method for CEO control and stabilization that circumvents the upper-state lifetime limitations of the gain medium, providing a higher control bandwidth and significantly improved noise characteristics of the

frequency-stabilized CEO signal.

[0007] Examples of realization in this field of endeavor can be found, among others, in the following references: · T. W. Hansch, "Nobel lecture: passion for precision," Rev Mod Phys 78, 1297-1309 (2006).

• S. Schilt, et at., " Fully stabilized optical frequency comb with sub-radian CEO phase noise from a SESAM-modelocked 1.5-μηη solid-state laser, " Opt. Express 19, 24171 -24181 (201 1). · C.-C. Lee, S. Suzuki, W. Xie, and T. R. Schibli, " Broadband graphene

electro-optic modulators with sub-wavelength thickness, " Optics Express 20, 5264-5269 (2012).

[0008] The method of modulating a light source by sharing a gain segment with another signal during amplification was demonstrated in other works for completely different purposes and using different amplification media, e.g., for optically-controlled wavelength conversion or switching in a semiconductor optical amplifier (SOA). The method was often referred to as Cross Gain Modulation (XGM). The following patents relate to such aspects in SOAs: IE20060331 , US2006092501 , EP1255157. These documents are not relevant for the invention reported here, since the claimed devices do not provide any mean to control or stabilize a parameter of the seed signal amplified in the considered semiconductor optical amplifier, such as its output power, but were rather used to modulate a CW signal with another signal having a different wavelength. The main purpose was for all-optical wavelength conversion or switching in optical telecommunications.

[0009] Optical frequency combs based on different modelocked laser technologies are known. The first frequency combs were based on

TkSapphire lasers, but nowadays fiber-based lasers (e.g., Er-fiber, Yb-fiber, Tm-fiber) are more widely used due to their simpler and more reliable architecture. Diode-pumped solid-state lasers (DPSSL) constitute another emerging technology for frequency combs. The traditional method to control and stabilize the CEO frequency is by feedback to the pump power of the modelocked laser, which is intrinsically limited in bandwidth by the dynamics of the laser cavity and the upper-state lifetime of the gain medium.

[0010] Intra-cavity loss modulators are known to provide a larger modulation bandwidth, which is in this case determined by the recovery time of the loss modulator itself. Known devices, described in

US2012327959, make use of a graphene EOM in the gain cavity. Other devices, described in EP3053266 and US2016226216 rely on opto-optical modulation (OOM) of a saturable absorber in the cavity of a DPSSL.

[0011] Gain modulation in a co-doped Er:Yb:glass diode-pumped solid- state modelocked laser has been reported in Opt. Letters 41 (2), 376-379 (2016) using stimulated emission induced by an external CW laser incident onto the gain medium. In this particular co-doped gain medium, a direct modulation of the intra-cavity power (used, e.g., for f_CEo stabilization) is primarily limited by the slow energy transfer between the Yb³⁺ ions and the Er³⁺ ions, and subsequently by the upper-state lifetime of the Er³⁺ ions. Modulating the optical power of a CW laser beam incident onto the same gain medium induces a modulation of the stimulated emission induced by the incident laser, which in turns affects the intra-cavity power of the modelocked laser. This modulation circumvents the slow Yb-Er energy transfer and offers a larger bandwidth than for direct modulation of the laser pump power. However, the method presents significant differences with the invention disclosed here. First of all, it is limited to the specific case of a co-doped Er:Yb bulk gain medium, while the present invention is applicable to any kind of fiber or waveguide gain medium that operates in the regime of high gain / high losses. The gain modulation via stimulated emission allows circumventing the slow energy transfer between the Yb and Er ions and enlarging the modulation bandwidth compared to direct pump power modulation, but it cannot overcome the limitation due to the gain upper-state lifetime, since the gain medium operates far from saturation. When used for CEO frequency stabilization, the gain

modulation via stimulated emission required a significant optical power incident onto the gain medium (typ. 500 mW), and could not achieve a fully independent locking. A complementary stabilization loop to the pump current of the modelocked laser was necessary to achieve a long-term stable stabilization.

Short summary of the invention

[0012] The present invention proposes a controlled laser or amplifier device and a method of controlling lasers and amplifier by a secondary control signal that is amplified in the laser or amplifier gain medium together with a main laser signal.

[0013] In a typical embodiment for a laser, the modulation is obtained by injecting a secondary signal that follows a common propagation path (either co-propagating or counter-propagating) into the gain medium and thus shares the gain of the laser. The secondary signal is amplified and reduces the output power of the main laser signal. The invention stems from the observation that this control mechanism requires a partially or completely saturated gain.

[0014] In a fiber or in a waveguide laser with fully or partially saturated gain (a common occurrence), modulation of the pump power leads to a low-pass filtered response of the output power or of the CEO frequency in the case of a modelocked laser frequency comb, with a cut-off frequency determined by the upper state lifetime of the gain medium that is, for ordinary gain media, in the range of some milliseconds down to hundreds of microseconds. The present invention avoids this limitation and provides modulation bandwidths exceeding 100 kHz and above 300 kHz in

embodiments.

[0015] A laser device according to the invention can be modelocked using a saturable absorber and dispersion compensation elements, as it is conventionally known. The modelocked laser emits pulses having a duration in the femtosecond or nanosecond range. Additional elements like tuners for the repetition rate can be included. The invention can be used to modulate, tune or stabilize the intra-cavity and output power of the laser or, through such modulation, control and stabilize the CEO frequency of the underlying frequency comb.

[0016] In a laser embodiment, the method of the invention can also be used for active modelocking by inducing a periodic loss at a given frequency. This can be the same as the resonator round-trip frequency, or an integer or fractional multiple thereof (harmonic modelocking).

[0017] In an amplifier, the seed signal and a secondary control signal that follows a common propagation path (either co-propagating or counter-propagating) into the gain medium are amplified simultaneously to control, modulate or stabilize the amplified seed signal. [0018] Particularly interesting is the use of high-bandwidth modulation for controlling the CEO frequency of a modelocked fiber or waveguide laser frequency comb. This invention can replace amplitude modulators like EOMs/AOMs for CEO or optical power control in many fiber systems such as fiber laser frequency combs, actively modelocked fiber lasers, passively modelocked fiber lasers, and low intensity noise fiber lasers.

[0019] In the context of the invention, the wording "saturated gain ", or " partially saturated gain ", or "completely saturated gain " relates to situations in which the gain medium cannot amplify an input seed signal with the same gain factor as the small signal gain (SSG) factor. If the total gain is lower than the SSG factor (typically <95% of the SSG factor), the gain is considered as partially saturated. If the amplification is fully completed (i.e., at more than 99%) before the signal reaches the end of the gain segment (typically after less than 99% of the length of the gain segment), then the gain is considered as completely saturated. [0020] [0021] Partially saturated gain is especially common in fiber and optical waveguide lasers and amplifiers that operate in a regime of high gain / high losses.

[0022] The invention offers the following benefits: · It requires a low-power light source as control signal (100 μ\Λ can be sufficient for some cases), which can be amplified in the gain medium. The control signal has usually a very low power compared to the actual laser or seed signal.

• The modulation amplitude can be tuned by a variety of means: by

changing the wavelength of the modulating light, by modifying the power ratio of the two amplified signals, by changing the amplitude of the modulating signal.

• The modulation bandwidth is not limited by the laser cavity dynamics, which depends in particular on the upper-state lifetime of the laser gain medium.

• The invention is more effective, especially for what bandwith and noise performance are concerned, than controlling a laser or amplifier by its pump current.

Description of the drawings

[0023] The invention will be better understood in relation with the figures in which:

Figure 1 shows a simplified schematics of a fiber laser according to one aspect of the invention.

Figure 2 is a simplified schematic of the proposed method used in a modelocked fiber laser. Figure 3 is a simplified schematics of a fiber amplifier according to one embodiment of the invention.

Figure 4 exemplifies a possible realization mode, without limitation for a fiber laser. Figures 5 and 6 plot an example of modulation transfer function in a fiber laser according to the present invention.

Figure 7 plots an example of frequency noise power spectral density (PSD) of the CEO beat obtainable using the present invention.

Figure 8 represents the RF spectrum of the CEO beat stabilized using the actuator of the present invention.

Description of embodiments and examples

[0024] Figure 1 shows a fiber laser in which the active gain medium is an optical fiber 32, suitably doped and pumped, and a resonator, in this case the ring fiber 35. The informed reader will appreciate that this is a highly simplified representation and that in a real application the laser may include several additional elements, not represented here.

[0025] In particular, optical devices according to the present invention may include dispersive and dispersion adjusting elements, including, but not limited to a fiber segment, and/or a diffraction grating, and/or a prism, and/or a Gires-Tournois Interferomenter (GTI) type mirror. [0026] The output of the laser is represented by terminal 30. Terminal 22 represents an auxiliary control signal generated by the auxiliary light source 23, which is injected in the gain medium, amplified at the same time as the laser radiation, and extracted at port 25. The auxiliary control signal can be co-propagating or counter-propagating with the main laser signal, the only difference being the method to in-couple and out-couple the auxiliary signal in and from the laser cavity. Insofar as the gain of the laser is saturated, the total power is ultimately limited by the pump power. The laser signal and the control signal share the total available power and this causes a gain transfer between the two. A suitable detector 29 is arranged to measure an output parameter, and its output signal, suitably processed, is fed back to the auxiliary source 23 for control or stabilization of the laser parameter. The auxiliary control signal can be used for controlling the intra-cavity power of the laser and, consequently, its output power, but the same scheme can be applied to the control and stabilization of many other parameters, for example the optical power (for intensity noise reduction). [0027] The auxiliary source may take any suitable form to generate a wavelength that can be amplified in the active medium. In particular, but not exclusively, it may include a semiconductor laser, an amplified spontaneous emission (ASE) source, a light emitting diode (LED).

[0028] Figure 2 represents the same device as figure 1 , with a

modelocking element 37, of any suitable known type. This variant is especially suitable for the generation of optical frequency combs, and the control unit 29 can be arranged to control or stabilize the CEO frequency of the frequency comb. The modelocking unit 37 may comprise one or more of a Nonlinear Polarization Rotation (NPR), SESAM (Semiconductor

Saturable Absorber Mirror), SBR (Saturable Bragg Reflector), a graphene or carbon nanotube saturable absorber, an AM (Active Modelocking) device.

[0029] Figure 3 relates to the case of a fiber optical amplifier, in which the seed (input) signal is injected at 29, and the amplified output is available at 30, while the auxiliary control signal is injected at 22 and extracted at 25. The auxiliary control signal can be co-propagating or counter-propagating with the main laser signal, the only difference being the method to in-couple and out-couple the auxiliary signal in and from the gain medium. The control unit can be arranged to control the output power of the amplified seed signal 30, or any other useful parameter. [0030] The present invention is not limited to fiber devices, ring lasers, and what is represented in these figures, but encompasses other embodiments, including dielectric or crystalline waveguide lasers, semiconductor waveguide lasers, and any applicable resonator kind that operates in a regime of partially or fully saturated gain. In the dielectric realization, the device may include waveguides and/or fibers comprising a dielectric solid doped with ions of Er, Yb, Tm, Ho, Nd, Pd, or a combination thereof, other forms of gain elements are possible in the frame of the invention.

[0031] In the examples proposed in the figures, the auxiliary signal propagates in the active medium in the opposite direction as the main optical signal, which allows discriminating between them by using an optical isolator. This is not an essential feature of the invention, however, and both co-propagating and counter-propagating realizations are possible and included in the scope of the invention.

[0032] In embodiments, the main signal and the auxiliary signal may be distinguished by their different polarization states. Polarization- maintaining components and fibers are used in this case, to prevent their mixing.

[0033] Figure 4 is a somewhat more detailed representation of an example of the invention. The laser comprises an Ytterbium-doped glass fiber 101 , one half-wave plate 105 and two quarter-wave plates 106 for polarization rotation, an isolator 108 to force uni-directional lasing, a diffraction grating pair 1 10 for dispersion compensation, a wavelength division multiplexer (WDM) 102 for coupling the pump light into the cavity, and a polarizing beam splitter (PBS) 107 for NPR rejection output. The fiber segments are spliced together (103) and the beams are collimated from the fiber or focused into the fiber using lenses 104. The beam in the cavity is reflected by silver mirrors 109, when necessary.

[0034] The Ytterbium-doped gain fiber is pumped by a telecom-grade single-mode diode laser 1 1 1, providing a continuous wave (CW) optical power of roughly up to 500 mW at a wavelength of 976 nm. The fiber laser has a repetition rate of 135 MHz and emits 160-fs pulses at a central wavelength of 1030 nm. The laser outputs 40 mW of average power.

[0035] The laser output is directed to a beam sampler 1 12 at an angle of 45 degrees, resulting in a reflection of around 8% of the s-polarization, followed by an optical isolator 108 to prevent subsequent back reflections from being fed back into the laser. The transmitted signal is amplified in a polarization maintaining (PM) fiber amplifier. The amplifier consists of a WDM combiner 1 15 made of PM fibers 1 14 and one Ytterbium-doped PM fiber 1 16. The signal is amplified up to 500 mW and is subsequently compressed using a pair of diffraction gratings 1 10. Note that a fiber amplifier might not be necessary in some configurations, when the laser output power is sufficient for the CEO detection setup. The diffraction gratings are included to compress the pulses to below 100-fs. While this is not an absolute necessity, having pulse duration shorter than 100 fs results in a higher coherence of the generated octave-spanning supercontinuum spectrum. Instead of diffraction gratings, fibers with suitable dispersion parameters can also be used to compress the pulses. In some cases, the laser can already emit sub-100 fs pulses that are suitable for coherent octave- spanning supercontinuum spectrum generation. [0036] The CEO beat can be detected by any known means. In our example, we use a photonic crystal fiber to generate an octave-spanning supercontinuum spectrum. Different types of nonlinear media can be used to generate the supercontinuum spectrum as well, such as integrated waveguides or highly nonlinear fibers. A standard f-to-2f interferometer 1 19 follows this to generate a CEO beat signal that is detected using an avalanche photodiode. The CEO beat is filtered using a bandpass filter 124, amplified in a radiofrequency amplifier 125 and fed into a phase detector 128 that compares its phase to a reference signal. The error signal is fed to a high bandwidth proportional-integrator-derivative (PID) servo-controller 1 18 and the correction signal is applied to the current driver 1 17 of the auxiliary control signal source 1 13 to stabilize the CEO frequency. [0037] The invention makes use of a semiconductor laser source to modulate the intra-cavity power of the fiber laser. In principle, a light source with a sufficient modulation bandwidth is the only necessity for the control signal source 1 13. This can be any laser, an LED, an ASE (amplified spontaneous emission) source or any other light source that serves this purpose. The polarization of the outgoing light is aligned to the s- polarization to efficiently inject it into the fiber laser cavity through the PBS 107. The beam is partially reflected at the beam sampler 1 12 and enters the cavity. The transmitted portion of the beam at the beam sampler 1 12 is dumped. The control signal, which is counter-propagating to the fiber laser signal in this particular realization, is amplified in the gain fiber along with the intra-cavity fiber laser signal. The amplified control signal is then blocked at the isolator 108, which prevents it oscillating in the cavity.

[0038] Due to the high optical losses occurring in a fiber laser cavity, a high gain is needed to sustain the laser operation. The high gain results in either a partial or a full saturation of the gain segment. This implies that a limited amount of energy is transferred to the laser signal. The additionally transmitted control signal sees some gain and steals energy from the limited reservoir, resulting in less energy for the original laser signal, and thus less gain. A balance between the laser signal and the control signal occurs. When the power of the control signal is increased, the power of the laser signal decreases accordingly. This implies that the modulation is imposed on the laser signal with an opposite phase.

[0039] An important parameter for the stabilization of the CEO frequency is the achievable modulation bandwidth. A large enough bandwidth is needed to correct for high frequency fluctuations that can contribute to the frequency noise of the CEO. The modulation bandwidth of semiconductor lasers, which can be used as control signal in this invention, is known to be very high, up to the gigahertz range. In the traditional method of modulating the pump laser of a fiber laser, the limiting factor arises mainly from the upper-state lifetime of the gain medium, which is longer than around 100 μ≤ for Yb fiber lasers, meaning that a modulation is efficient only at frequencies below 10 kHz. When modulating the seed signal of a typical saturated single-channel fiber gain, the inverse is observed, i.e., only the modulation frequencies higher than 10 kHz are efficiently transferred. The present invention bypasses the limitation imposed by the gain and upper-state lifetime. Since these gain limitations act on the combination of the fiber laser and control signals, the two signals do not suffer individually from these limitations. A balance occurs between the laser and control signals. Forcing the fiber laser signal to share its gain segment with another signal (the control signal) is the basis of the proposed method. Figures 5 and 6 show an example of modulation transfer function from the power of the control signal to the output power of the fiber laser. The transfer function has a cut-off frequency of around 1.2 MHz both in amplitude (-3 dB) and phase (-90°). The high modulation bandwidth of the method allows for straightforward tight locking of the CEO beat. [0040] The frequency noise power spectral density of the CEO beat signal stabilized using the present invention is shown in Figure 7. The servo bump at 300 kHz indicates the stabilization bandwidth of the feedback loop. This value is much higher than achievable using the traditional method of pump power modulation in the same laser and results in a residual integrated phase noise of 85 mrad (integrated between 1 Hz and 100 kHz). The stabilized CEO beat signal is shown in Figure 8. A tight lock of the CEO beat is easily achieved using the present invention, as shown by the coherent peak observed in the center of the RF spectrum.

[0041] The choice of the gain material does not affect the applicability of the invention. Erbium, Ytterbium, Thulium, Holmium, Neodymium, and Praseodymium are among the most commonly used fiber and waveguide dopants. However, the invention is not limited to these doping ions. For each doping material, the wavelength of the control signal source must be selected appropriately, depending on the emission cross section of the gain material. As the control signal is rather amplified (and not absorbed) in the gain fiber, the modulation is transferred to the fiber laser signal. As the control signal power increases, it steals gain from the fiber laser signal, which sees less gain. As a result, the modulation is transferred to the laser signal with an inverse sign.

[0042] The minimum optical power that is needed for the control signal source depends on the system and needs. As an example, a ratio of 1 to 100 between the optical power of the control signal source and the intra-cavity laser signal results in a modulation depth of around 1 %. This is more than sufficient for the stabilization of the CEO beat. Several factors, such as the wavelength of the fiber laser, the wavelength of the control signal source, the co- or counter-propagation of the signals, can affect this ratio. [0043] The control signal needs to propagate in the same gain medium as the laser signal. In the demonstration presented here, this is achieved by counter-propagating the control signal to the fiber laser signal. Following the gain fiber segment, the control signal is blocked by the optical isolator. In another embodiment, the fiber laser signal and the control signal can be combined by polarization. The mixing of the two polarizations can be prevented by the use of polarization maintaining fibers either in a co- propagating or a counter-propagating configuration. After the gain segment, the control signal can be suppressed at a polarization blocking element or separated by a polarization beam splitter. In still another embodiment, the fiber laser signal and the control signal can be combined by regular (non PM) components in a counter-propagating configuration.

[0044] Due to the nature of the present invention acting on the gain segment of the fiber laser, the used modelocking mechanism does not affect the working principle of the present invention. Different

modelocking techniques can be employed in the fiber laser, such as NPR, SESAM, carbon nanotubes, or graphene saturable absorbers. The different pulse durations, pulse shapes or operating regimes and dispersion profiles do not play a role in the working principle of the present invention either. However, the saturable absorber and the dispersion of the cavity can play a significant role on the laser performance. [0045] In another embodiment, the present invention can be used to stabilize the intra-cavity power of a fiber laser and thus its output power. In this case, the output power of the laser is compared to a reference value. The fiber laser can run in CW or in modelocked operation. [0046] In another embodiment, the present invention can be used to actively modelock a fiber laser using amplitude modulation. A sine-wave modulation can be applied through the control signal to a CW fiber laser. The frequency of the applied modulation is determined by the resonator length of the fiber laser. Using this technique, the fiber laser can be fundamentally- or harmonically-modelocked.

[0047] The use of the present invention for active modelocking can surpass other modulator solutions such as acousto-optic or electro-optic modulators.

[0048]

Reference numbers

[0049]

22 input of the auxiliary signal

23 auxiliary source

25 output of the auxiliary signal

29 detector

30 laser or amplifier output

32 doped fiber

35 ring fiber/resonator

37 modelocking device

100 undoped fiber

101 Yb-doped fiber

102 wavelength division multiplexer (WDM)

103 splice

104 lens

105 half-wave plate

106 quarter-wave plate

107 polarizing beam splitter

108 isolator

109 silver mirror

1 10 diffraction grating

1 1 1 single-mode diode laser, source

1 12 wavelength division multiplexer (WDM)

1 13 control signal source

1 14 Polarization maintaining (PM) fiber 1 15 combiner

1 16 Yb-doped fiber

1 17 current driver

1 18 high-bandwidth controller

1 19 f-2f interferometer

124 low-pass filter

125 amplifier

128 phase detector

Claims

1. A laser or amplifier optical device comprising a gain element (101, 32), a pump unit operatively arranged to pump the gain element into a condition of population inversion, wherein it can amplify a main optical signal with an amplification gain that is at least partially saturated, the optical device including an auxiliary input (22) optically connected to an auxiliary source (23, 1 13) generating an auxiliary optical signal that is propagated at least in a part of the gain element together with the main optical signal, and with a wavelength that is amplified in the gain element (101 , 32), the optical device further comprising a control unit (1 18) operatively arranged to control the auxiliary source (23, 1 13).

2. The optical device of the preceding claim, being a modelocked laser generating an optical frequency comb, in which the control unit is operatively arranged to stabilize, modulate or tune the carrier envelope offset frequency of the frequency comb.

3. The optical device of any one of the preceding claims, wherein the auxiliary source is any of: a laser, in particular a semiconductor laser, an amplified spontaneous emission (ASE) source, a light emitting diode (LED).

4. The optical device of any one of the preceding claims, wherein the gain element is a solid material doped with ions of Er, Yb, Tm, Ho, Nd, Pd, or a combination thereof.

5. The optical device of any one of the preceding claims, comprising a detector (29, 1 19) with an output responsive to a parameter of the main optical signal, the output of the detector acting on a control unit (1 18) operatively arranged to control the auxiliary source (23, 1 13) in order to control the parameter.

6. The optical device of any one of the preceding claims, in which the control unit is operatively arranged to control and/or stabilize the intra- cavity power and/or output power, and/or the carrier-envelope offset, and/or reduce a noise of the optical device.

7. The optical device of any one of the preceding claims, being a dielectric or crystalline waveguide laser in which the gain element is a doped dielectric or crystalline waveguide or optical fiber, or a semiconductor waveguide laser, in which the gain element is a semiconductor waveguide.

8. The optical device of any one of the preceding claims, being a laser source, wherein the gain element is arranged in a cavity for amplifying multiple times the main optical signal.

9. The optical device of any one of the preceding claims, wherein the control signal is propagating in the gain element in the opposite direction as the main optical signal, or in the same direction as the main optical signal.

10. The optical device of any one of the preceding claims including a modelocking mechanism, preferably comprising one or more of: a SESAM (Semiconductor Saturable Absorber Mirror), an SBR (Saturable Bragg Reflector), a carbon nanotube or graphene saturable absorber, an AM (Active Modelocking) device.

1 1.The optical device of any one of the preceding claims including a dispersion adjusting element that may include a fiber segment, and/or a diffraction grating, and/or a prism, and/or a Gires-Tournois Interferomenter (GTI) type mirror.

12. The optical device of any one of the preceding claims, being a

modelocked laser with a repetition rate between 1 kHz and 500 GHz, and/or pulse width between 1 fs and 10 ps.

13. The optical device of any one of claims 1 , 3 to 6, 9, , being an amplifier, in which the gain element has an input receiving a seed signal and an output generating an amplified signal, and the control unit is operatively arranged to control and/or stabilize the power and/or reduce the noise of the amplified signal.

14. The optical device of any one of the preceding claims, including polarization-maintaining elements.